This invention relates generally to polymerization of olefins and more particularly to a new process and apparatus for producing olefin polymers by polymerizing .alpha.-olefins in gaseous phase.
Throughout this disclosure, including the appended claims, the terms "polymerization" and "polymers" are intended to include "homopolymerization", "copolymerization" and "homopolymers", "copolymers", respectively.
Various methods of producing crystalline high-molecular polymers by catalytic polymerization of .alpha.-olefins such as ethylene and propylene have been proposed. The catalysts used in these methods, in general, are solids, and of these, various catalysts of excellent stereospecificity are known.
One typical example of these methods is that wherein an .alpha.-olefin is caused to contact a polymerization catalyst dispersed in a liquid organic dispersing agent. It may be considered that the .alpha.-olefin dissolves in this dispersing agent and thereby contacts the catalyst. Processes based on this method in which a so-called Ziegler type catalyst is used as the polymerization catalyst are being industrially practiced in some instances because of several advantages thereof such as facility in producing olefin polymers of good crystallinity and stereospecificity and facility in controlling the polymerization temperature.
On the other hand, however, the practice of this method on an industrial scale is accompanied by various problems such as the necessity of using a large quantity of a purified dispersing agent and the necessity of a chemical treatment step for removing catalyst residue from the polymer formed since the yield with respect to the catalyst, that is, the ratio of the quantity of the olefin polymer formed to the quantity of the catalyst or the catalyst component used, is not very high.
In addition to the above described so-called slurry polymerization method, a method wherein polymerization is accomplished by causing an .alpha.-olefin in a gaseous phase to contact a catalyst in the substantial absence of a liquid dispersing agent. This method is disclosed in the specification of Japanese Patent Publication Nos. 1,895/1958 and 9,892/1959. A process based on this method is, of course, free of the problems accompanying the use of a dispersing agent and can be considered to be a very economical process.
In this gaseous-phase process, the polymerization catalyst, which is generally a solid is used directly in its solid state, in a state wherein it is dispersed in a small quantity of a dispersing agent, in a state wherein it is borne on a carrier comprising a granular olefin polymer, an inorganic substance such as silica, alumina, titanium oxide, or calcium carbonate, or some other substance, or in some other suitable state and caused to contact the starting-material olefin in gaseous state. It can be easily surmised that the state of gas-solid contact in this process has a great effect on this polymerization process.
Accordingly, a number of proposals have heretofore been made relative to this point. For example, in one proposed process, a fludiized bed is formed by a solid phase comprising particles of the olefin polymer formed and the catalyst and by the starting-material olefin.
By this process, however, a gas flow velocity of at least 5 to 6 cm/second is ordinarily necessary for causing the above mentioned solid phase to become fluid, whereby not only is the supply of a large quantity of the starting-material olefin necessary, but fine solid-phase particles (the inclusion in which of the catalyst must be taken into consideration) accompany the starting-material olefin rising in the fluidized bed and are thereby discharged out of the system. Furthermore, in addition to regions of rapid flow, regions of very slow flow readily occur in the fluidized bed. In the latter regions of sluggish flow, there is a tendency of the olefin polymer formed to collect and solidify to become lumps, whereby continuous operation over a long period necessary for industrial practice is not possible.
As an attempt to solve the above described problems accompanying this fluidized-bed process, a process wherein the flow velocity of the gas sent toward the fluidized bed is lowered by applying to the fluidized bed an auxilliary mechanical agitation or vibration thereby to prevent the scattering of the fine solid phase has been proposed. However, even by this process, the above described problems cannot be completely solved.
More specifically, probably because of the decrease in the gas flow velocity, the porous or sieve-like partition for preventing fluid solids from dropping within the reaction vessel or the gas suction inlet is clogged with fine solid particles whereby the gas dispersion becomes irregular, or the flow of the gas is stopped. The resulting ununiform state of the fluidization gives rise to irregularities in the fluidized bed temperature or to agglomeration and solidification of olefin polymers.